1. Field of the Invention
The present invention relates to an image forming apparatus such as copiers, printers, facsimiles and multi-functional peripherals thereof. Particularly, the invention relates to an image forming apparatus in which after transfer of a toner image formed on a rotatably driven photoreceptor to an intermediate transfer belt, a surface of the photoreceptor is destaticized by irradiating the surface of the photoreceptor with light from a static eraser including a plurality of light emitting elements arranged in an axial direction of the photoreceptor.
2. Description of the Related Art
The image forming apparatuses such as such as copiers, printers, facsimiles and multi-functional peripherals thereof are conventionally adapted to perform: an operation by a charger unit which electrically charges the surface of a rotatably driven photoreceptor; an operation by a latent image forming unit which forms an electrostatic latent image on the photoreceptor surface by exposing the charged surface of the photoreceptor to light according to image information; an operation by a developing unit which forms a toner image on the photoreceptor surface by supplying toner to the electrostatic latent image formed on the photoreceptor surface; an operation by the static eraser which removes the static charge remaining on the photoreceptor surface after transfer of the toner image thus formed on the photoreceptor surface to an intermediate transfer belt or recording medium; and an operation by the charger unit which charges again the destaticized surface of the photoreceptor.
A device which includes a plurality of light emitting elements such as LEDs arranged in the axial direction of the photoreceptor and is adapted to destaticize the photoreceptor surface by irradiating the photoreceptor surface with light from the individual light emitting elements thereof is widely used as the above static eraser.
In a case where the surface of the photoreceptor is destaticized by irradiating the photoreceptor surface with light from the individual light emitting elements arranged in the axial direction of the photoreceptor, the intensity of light applied to the photoreceptor surface at a portion opposite to the light emitting element is higher than that of light applied to the photoreceptor surface at a portion opposite to space between the light emitting elements and hence, the surface of the photoreceptor is unevenly destaticized in the axial direction thereof.
When the unevenly destaticized surface of the photoreceptor is electrically charged by the charger unit, the photoreceptor surface is unevenly charged, which results in density unevenness and the like of formed images.
More recently, a patent document 1 (Japanese Unexamined Patent Publication No. H06-230658) has proposed an apparatus which is equipped with the static eraser including a plurality of light emitting elements arranged with predetermined spacing in the axial direction of the photoreceptor and which operates an oscillating cam or the like to oscillate the light emitting elements in a moving direction of the photoreceptor surface. At turn-on of the light emitting elements, optical axes of the light emitting elements are displaced to a downstream side of the moving direction of the photoreceptor surface. Just before turn-off of the light emitting elements, on the other hand, the optical axes of the light emitting elements are displaced to an upstream side of the moving direction of the photoreceptor surface. The photoreceptor surface is prevented from being inadequately destaticized at the portion opposite to space between the light emitting elements in the moving direction of the photoreceptor surface just after turn-on or just before turn-off when the light quantity of the light emitting elements transiently vary. Thus is provided a clear linear borderline of destaticization.
However, the apparatus disclosed in the above patent document 1 requires a mechanism such as the oscillating cam for displacing the optical axes of the light emitting elements to the downstream side and the upstream side of the moving direction of the photoreceptor surface at turn-on of the light emitting elements and just before turn-off of the light emitting elements. This leads to a problem that the apparatus is increased in costs and size. Further, in a case where the intensity of light applied to the photoreceptor surface varies between the portion opposite to the light emitting element and the portion opposite to space between the light emitting elements so that the photoreceptor surface is unevenly destaticized, the apparatus that is simply adapted to displace the optical axes of the light emitting elements to the downstream side and the upstream side of the moving direction of the photoreceptor surface as described above has difficulty in properly detecting such a destaticization unevenness and fully eliminating the destaticization unevenness.
Further, a patent document 2 (Japanese Unexamined Patent Publication No. 2009-175675) has proposed an apparatus in which, in an image non-forming period, a photoreceptor drum is charged by applying a predetermined voltage to a contact charging member, a region of the photoreceptor surface is exposed to light by turning on a standard exposure device adjusted to a predetermined light quantity without turning on a pre-charge exposure device, and a first charging current value of the current through the region recharged by the contact charging member is detected by a charging current detection device; in which, in the image non-forming period, the photoreceptor drum is charged by applying the predetermined voltage to the contact charging member, the region of the photoreceptor surface is exposed to light by turning on the pre-charge exposure device without turning on the standard exposure device, and a second charging current value of the current through the region recharged by the contact charging member is detected by the charging current detection device; and in which a control unit determines a light quantity of the pre-charge exposure device in an image forming period based on the first and second charging current values, thus properly optimizing the light quantity of the pre-charge exposure device in the image forming period without using surface potential detection means.
However, the apparatus of the above patent document 2 requires an additional circuit for setting the light quantity of the pre-charge exposure device because the control unit controls the light quantity of the pre-charge exposure device in the image forming period based on the first and second charging current values. What is more, in a case where the intensity of the light applied to the photoreceptor surface varies between the portion opposite to the light emitting element and the portion opposite to space between the light emitting elements so that the photoreceptor surface is unevenly destaticized, the apparatus has difficulty in properly detecting such a destaticization unevenness and fully eliminating the destaticization unevenness.
Further, a patent document 3 (Japanese Unexamined Patent Publication No. 2007-232881) has proposed an apparatus in which an electrostatic latent image is formed on a photoreceptor by charging means charging the photoreceptor surface to a primary charge potential higher than an image forming potential, followed by exposure means exposing an image non-forming area to laser of a surface-potential control power to lower the surface potential to an image forming potential while exposing an image forming area to laser of an image forming power. The image forming apparatus is adapted to perform image formation without being equipped with an additional static erasing member such as a preliminary charging member or light emitting diode.
However, the apparatus of the patent document 3 has problems that load on the photoreceptor is increased because the photoreceptor surface is charged to the primary charge potential higher than the image forming potential and that the apparatus is complicated and increased in costs because the exposure means requires control and mechanism to vary the power of laser to be applied to the image non-forming area and to be applied to the image forming area. Since the static erasing member such as the light emitting diode is not added to the apparatus, the apparatus per se does not involve the problem that the intensity of light applied to the photoreceptor surface varies between the portion opposite to the light emitting element and the portion opposite to space between the light emitting elements and the destaticization unevenness on the photoreceptor surface results.